The present invention relates to apparatus for deriving a feedback control signal in a thermal system wherein an electrical element supplies heat to maintain stabilized conditions. Such apparatus is commonly known in the art but all known apparatus are either nonfunctional at very high temperatures, such as 2500 degrees centigrade or produce substantial control error as electrothermal parameters vary such as the resistance of the heating element. Typical of the nonfunctional apparatus is that in which a temperature feedback signal is derived by using a thermocouple that is located within the controlled zone of the thermal system. Prior art apparatus wherein feedback control signals are derived within electrothermal systems from either the current through, or the voltage across, or the power input to the heating element thereof are typical of those with which substantial control error is encountered.